Field of the Invention
The invention relates to an emitter.
The lifetime of a thermal electron emitter in an X-ray tube (surface emitter, filament emitter) is in the first instance determined by the thermally induced evaporation of the emitter material used, generally tungsten. Hence, higher lifetimes can be achieved by either a higher material thickness of the emitter and/or a lower emitter temperature. In such cases, an increased thickness causes a linear increase in the lifetime, while the influence of the temperature on the evaporation of the material has an exponential dependence.
A reduction of the emitter temperature requires an enlargement of the emission surface and hence the emitter surface. Hence, greater effort is generally required to focus the electrons emitted to form an electron beam.
Increasing the material thickness in the region of the emission surface (thicker surface emitter plate, larger filament wire diameter) requires higher heating currents and results in higher thermal inertia. In the case of surface emitters with connecting legs (non-directly welded surface emitters), it is only possible to bend the connectors up to a specific emitter thickness. Hence, limits are placed on an increase in the material thickness.
German patent DE 27 27 907 C2 describes a surface emitter containing a basic unit with a rectangular emitter surface. The basic unit or the emitter surface has a layer thickness of from about 0.05 mm to about 0.2 mm and is, for example, made of tungsten, tantalum or rhenium. In the case of tungsten, it is also known to carry out potassium doping. The surface emitters produced in a rolling process have incisions which are arranged in alternation from two opposite sides transverse to the longitudinal direction. During the operation of the X-ray tubes, heating voltage is applied to the surface emitter of the cathode, wherein heating currents from about 5 A to about 20 A flow and electrons are emitted and accelerated in the direction of an anode. X-radiation is generated in the surface of the anode when the electrons strike the anode.
According to German patent DE 27 27 907 C2, the shape, length and arrangement of the lateral incisions enable special configurations of the temperature distribution to be achieved in the surface emitter since the heating of a body heated by current passage therethrough depends on the distribution of the electrical resistance across the current paths. Hence, less heat is generated at points at which the electrically active planar cross section of the surface emitter is greater than at points with a smaller cross section (points with a greater electrical resistance).
The surface emitter disclosed in German patent DE 199 14 739 C1 in turn contains a basic unit made of rolled tungsten plate and in this case has a circular emitter surface. The emitter surface is divided into conducting tracks extending in spirals that are spaced apart from one another by serpentine incisions.
In addition, published, non-prosecuted German patent application DE 10 2014 211 688.0 describes a surface emitter containing a monolithic basic unit. Selectively increasing the thickness of the basic unit at temperature-critical points causes local drops in the temperature at these points.
German patent DE 10 2009 005 454 B4, corresponding to U.S. Pat. No. 8,227,970, discloses an indirectly heated surface emitter. The surface emitter contains a primary emitter and a heating emitter spaced apart therefrom both having a circular primary surface. The primary emitter contains an unstructured primary emission surface, i.e. a homogeneous emission surface without slots. The directly heated heating emitter contains a structured heat emission surface, i.e. an emission surface with slots or serpentine tracks. The primary emission surface and the heat emission surface are aligned substantially parallel to one another and insulated from one another.
A cathode with a filament emitter (incandescent filament) is, for example, known from published, non-prosecuted German patent application DE 199 55 845 A1.